1. Technical Field
The present invention relates to a method of manufacturing a semiconductor device usable as a sensor, such as an acceleration sensor and an angular velocity sensor, that comprises a movable section formed in a semiconductor layer of a semiconductor substrate and displaceable in response to a dynamic quantity acting thereon.
2. Related Art
A conventional semiconductor device of this type has been for example disposed by Japanese Patent Application Laid-open No. 11-274142. This semiconductor device uses an SOI substrate formed as a semiconductor substrate, in which the substrate is composed of a first silicon layer (a first semiconductor layer), an oxide layer (insulation layer) layered on the first silicon layer, and a second silicon layer (a second semiconductor layer) layered on the silicone oxide layer.
For manufacturing the above semiconductor device, a trench to demarcate a movable section is formed. Specifically, a first step of dry etching is performed so that the trench is formed to reach the oxide layer from the surface of the second silicone layer. Then a second step of dry etching is performed to apply etching ions to a partial region of the second silicon layer, which is located laterally to the bottom of the trench. This etching allows the partial region of the second silicone layer to be removed, thus providing a movable section formed by the remaining area of the second semiconductor layer.
The foregoing manufacturing step of the movable section will now be detailed. Through the second step of try etching, which is done after forming the trench, a partial region of the insulating film, which is located at the bottom of the trench, is charged at first. This charge causes etching ions to be repelled during the dry etching continued, so that etching ions travel as they change their courses from the depth direction of the trench to the lateral direction orthogonal thereto. Accordingly, the etching ions are forced to impinge onto a partial region of the second semiconductor layer, which is located on the sides of the bottom. The partial region of the second semiconductor layer is thus subjected to the etching so that it is removed, providing a movable section above the insulation layer.
In the case of the technique disclosed by the above conventional patent publication, an etched depth is detected by utilizing movable sections formed dedicatedly to detecting the etched depth. The foregoing charging is used to stick (bond) the adjacent depth-detecting movable sections to each other.
However, the estimation of the foregoing manufacturing technique carried out by the inventors showed two problems.
One problem is that a sticking phenomenon may arise among 15 members of the semiconductor substrate. In addition to the insulation layer at the trench bottom, the dry etching carried out after forming the trench charges not only the movable section responsible for a sensing part of the device but also the second semiconductor layer and insulation layer which are located oppositely to the trench via the movable section. This charging brings about an imbalanced charging distribution among such various members, so that there is a fear that the members are partly or entirely subjected to sticking phenomena to their surrounding members.
The other problem also relates to the sticking phenomenon due to deposits. During the performance of the dry etching to remove regions of the second semiconductor layer, which are located on the sides of the trench bottom, deposits that appear due to the etching ions may adhere on a region of the movable section, which is opposed to the insulation layer. The deposits are composed of for example polymers including fluorine, carbon and oxygen.
If the deposits adhere on the movable section, as stated above, both the movable section and the insulation layer are likely to be stuck to each other at the locations of the deposits. One reason is that the deposits make it narrow the distance between the movable section and the insulation layer.
In cases where the movable section which should operate as the sensing part to the device fall into the stuck state on account of the foregoing charging or deposits, it is no longer possible to make the movable section operate normally as the sensing part of the device.
The present invention, which has been made with due consideration to the foregoing drawbacks, is directed to an object of providing a method of manufacturing a semiconductor device that has a movable section formed by removing the surrounding parts of the bottom of a trench through dry etching is prevented from sticking to the surroundings.
In order to accomplish the above object, the present invention provides a method of manufacturing a semiconductor device having a semiconductor substrate. In the semiconductor substrate, an insulation layer is layered on a first semiconductor layer and a second semiconductor layer is layered on the insulation layer. A movable section is formed in the second semiconductor layer and displaceable in response to a dynamic quantity to be applied.
As one aspect, the manufacturing method comprises the steps of preparing the semiconductor substrate; forming a trench to demarcate the movable section within the prepared semiconductor substrate so that the trench reaches the insulation layer form a surface of the second semiconductor layer; and forming the movable section by performing dry etching on the trench-formed semiconductor substrate, during which the dry etching the insulation layer located at a bottom of the trench is part of charged to force etching ions of the dry etching to impinge onto the second semiconductor layer located laterally to the bottom of the trench, thus the laterally located part of the second semiconductor layer being removed to form the movable section. In addition, electric charges caused due to the charging of the insulation layer during the movable-section forming step are removed from at least one of the movable section, a region, which faces the movable section, of the second semiconductor layer, and a region, which faces the movable section, of the insulation layer.
Hence, even when the charging of the insulation layer carried out during the movable-section forming process causes various members other than the insulation layer, electric charges appearing on the members are removed. Such members include the movable section, a region, which faces the movable section, of the second semiconductor layer, and/or a region, which faces the movable section, of the insulation layer. The removal of the electric charges prevents the movable section from sticking to its surroundings.
It is not always required that the electric charges on the members be removed completely. As long as the movable section is prevented from sticking to its surroundings, a moderate level of removal of the electric charges is still enough.
By way of example, the electric charges may be removed by intermittently performing the dry etching during the movable-section forming step. In this case, during a certain time of period following a period of time for the dry etching, no dry etching is carried out, thus being left to stand so that the electric charges disappear. Since the electric charges appear every time the dry etching is performed intermittently are necessarily subject to a period of time for their disappearances. Thus, when the entire dry etching process is completed, electric charges are hardly left in the substrate, thereby preventing the movable section from sticking to its surrounding members.
In addition, for performing the dry etching in the intermittent manner as stated above, the electric charges may be removed by providing the electric charges with ions to neutralize the electric charges during each of the intermittent periods of time with no dry etching. This leads to the repeated alternating steps for both the performance of the dry etching and the administration of the neutralizing ions. The electric charges can therefore be removed in a more effective way. Negative ions such as oxygen ions are one example of such neutralizing ions.
Another technique for removing the electric charges is to leave the movable-section-formed semiconductor substrate during a period of time determined so that the electric charges disappear, after the movable-section forming step. Although electric charges are accumulated immediately after the movable-section forming step, that is, the step of the dry etching, the electric charges will disappear, with reliability, during a certain period of time for being left to stand. The movable section is thus prevented from sticking to its surrounding members.
Still another technique for removing the electric charges is providing the electric charges with ions to neutralize the electric charges after the movable-section forming step. Like the above, this technique is also effective in removing the electric charges, thus preventing the sticking phenomenon of the movable section.
There is sill another removing technique of the electric charges. That is, the electric charges can be removed, during the trench forming step, by placing an electrically conductive member on either of a region, which provides at least the movable section, of a surface of the second semiconductor layer or a region, which provides a partial region facing the movable section, of the second semiconductor layer. The electric charges to be appear on the movable section and/or a region, which faces the movable section, of the second semiconductor layer are able to disappear via the conductive member with effectiveness, thereby being effective in preventing or suppressing the sticking actions of the movable section.
For example, the conductive member may be used in common with a mask member for forming the trench.
In order to suppress or prevent the movable section from sticking to its surroundings due to deposits, not the foregoing electric charges, the following configurations are provided as another aspect of the present invention.
A first configuration is to clean up a region, which faces the insulation layer, of the movable section after the trench and movable-section forming steps. By this configuration, deposits on a region, which faces the insulation layer, of the movable section during the movable-section forming step can be taken out by the cleaning. The sticking phenomenon of the movable section, which is caused by the deposits, can therefore be avoided.
The removal of the deposits is also effective in securing smooth movements of the movable section, which is favorable to the characteristics of the semiconductor device. In detail, the above cleaning process can be performed in a vapor phase containing ions to remove organic substances, such as oxygen ions.
Still another aspect of the present invention is provided by the following configurations. Such aspect is directed to the suppression of sticking of the movable section due to the charging phenomenon caused during the movable-section forming step.
Practically, after both the trench and movable-section forming steps, a roughing step is carried out to roughen at least one of facing surfaces (13a, 24a) of both the movable section and the insulation layer, the facing surfaces are opposed to each other between the movable section and the insulation layer face. This roughing process is effective in reducing the contact areas of both the facing surfaces. In cases where the foregoing charging brings about a contact between the movable section and the insulation layer, a reduction in the contact areas allows a slight restoration force to separate both of the members from each other, thus preventing the sticking action of the movable section.
Preferably, in cases where the facing surface of the movable section is roughened, the roughing step is performed in a vapor phase containing ions to primarily etch the second semiconductor layer. In contrast, in cases where the facing surface of the insulation layer is roughened, the roughing step is performed in a vapor phase containing ions to primarily etch the insulation layer. Such ions may be for example fluorine ions.